Strength characterization of knee flexor and extensor muscles in Prader-Willi and obese patients

Background: despite evidence of an obesity-related disability, there is a lack of objective muscle functional data in overweight subjects. Only few studies provide instrumental strength measurements in non-syndromal obesity, whereas no data about Prader-Willi syndrome (PWS) are reported. The aim of our study was to characterize the lower limb muscle function of patients affected by PWS as compared to non-syndromal obesity and normal-weight subjects. Methods: We enrolled 20 obese (O) females (age: 29.1 +/- 6.5 years; BMI: 38.1 +/- 3.1), 6 PWS females (age: 27.2 +/- 4.9 years; BMI: 45.8 +/- 4.4) and 14 healthy normal-weight (H) females (age: 30.1 +/- 4.7 years; BMI: 21 +/- 1.6). Isokinetic strength during knee flexion and extension in both lower limbs at the fixed angular velocities of 60 degrees/s, 180 degrees/s, 240 degrees/s was measured with a Cybex Norm dynamometer. Results: the H, O and PWS populations appear to be clearly stratified with regard to muscle strength.: PWS showed the lowest absolute peak torque (PT) for knee flexor and extensor muscles as compared to O (-55%) and H (-47%) (P=0.00001). O showed significantly higher strength values than H as regard to knee extension only (P=0.0014). When strength data were normalised by body weight, PWS showed a 50% and a 70% reduction in PT as compared to O and H, respectively. Knee flexors strength values were on average half of those reported for extension in all of the three populations. Conclusion: the novel aspect of our study is the determination of objective measures of muscle strength in PWS and the comparison with O and H patients. The objective characterization of muscle function performed in this study provides baseline and outcome measures that may quantify specific strength deficits amendable with tailored rehabilitation programs and monitor effectiveness of treatments.